Back to EveryPatent.com
United States Patent |
5,238,860
|
Sawada
,   et al.
|
August 24, 1993
|
Semiconductor device having different impurity concentration wells
Abstract
A semiconductor device comprises an N-type semiconductor substrate, a first
P-type well formed in the semiconductor substrate, a second P-type well
formed adjacent to the first P-type well in the semiconductor substrate,
the surface impurity concentration of the second P-type well being set
lower than that of the first P-type well, a DRAM memory cell structure
formed in the first P-type well, and an MOS transistor structure formed in
the second P-type well to function in combination with the memory cell
structure.
Inventors:
|
Sawada; Shizuo (Yokohama, JP);
Fujii; Syuso (Kawasaki, JP);
Ogihara; Masaki (Yokohama, JP)
|
Assignee:
|
Kabushiki Kaisha Toshiba (Kawasaki, JP)
|
Appl. No.:
|
816565 |
Filed:
|
January 3, 1992 |
Current U.S. Class: |
438/210; 257/E21.63; 257/E27.086; 257/E27.092; 438/220; 438/241 |
Intern'l Class: |
H01L 021/265; H01L 021/70 |
Field of Search: |
437/43,47,48,51,49,52,152,153,154,60,919,192
357/23.6,40
257/302,303,304,409
|
References Cited
U.S. Patent Documents
4403395 | Sep., 1983 | Curran | 437/48.
|
4516313 | May., 1985 | Turi et al. | 437/52.
|
4637125 | Jan., 1987 | Iwasaki et al. | 437/152.
|
4672410 | Jun., 1987 | Miura et al. | 257/302.
|
4673962 | Jun., 1987 | Chaterjee et al. | 254/68.
|
4697332 | Oct., 1987 | Joy et al. | 148/DIG.
|
5108944 | Apr., 1992 | Shirai et al. | 437/149.
|
Foreign Patent Documents |
53-23577 | Apr., 1978 | JP | 437/56.
|
60-211867 | Oct., 1985 | JP | 437/57.
|
Other References
"An Experimental 4Mb CMOS Dram", Furuyama, et al., 1986 IEEE International
Solid-State Circuits Conference, ISSCC 86/Feb. 21, 1986.
"An Experimental 1-Mbit BiCMOS Dram", Kitsukawa, et al., IEEE Journal of
Solid-State Circuits, vol. SC-22, No. 5, Oct. 1987.
"Advanced BiCMOS Technology for High Speed VLSI", Ikeda., IEDM 86, IEEE
1986.
"Bipolar CMOS Merged Structure for High Speed M Bit DRAM", Kobayashi, et
al., IEDM 86, IEEE 1986.
"Physics and Technology of Semiconductor Devices", A. S. Grove, Fairchild
Semiconductor, John Wiley and Sons, Inc., p. 209 (Date Unknown).
"Physics of Semiconductor Devices", S. M. Sze, John Wiley and Sons, pp.
192-193 and 196-197 (Date Unknown).
|
Primary Examiner: Chaudhuri; Olik
Assistant Examiner: Trinh; Loc Q.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner
Parent Case Text
This is a division of application Ser. No. 07/609,076, filed Nov. 7, 1990
now U.S. Pat. No. 5,079,613, which is a continuation of first-filed
application Ser. No. 07/216,045, filed Jul. 7, 1988, now abandoned.
Claims
What is claimed is:
1. A method of forming a semiconductor device with a second conductivity
type semiconductor substrate comprising the steps of:
forming a first well of a first conductivity type with a surface impurity
concentration in the semiconductor substrate;
forming, separately from the first well, a second well of the first
conductivity with a lower surface impurity concentration than the impurity
concentration of the first well;
forming, separately from the semiconductor substrate, a third well of the
second conductivity type in the second well;
forming, separately from the first well and the second well, a fourth well
of the first conductivity type with a lower impurity concentration than
the impurity concentration of the first well and a higher impurity
concentration than the impurity concentration of the second well;
forming, separately from the first well, the second well, and the fourth
well, a fifth well of the second conductivity type in the semiconductor
substrate with a higher majority carrier concentration than the majority
carrier concentration of the third well;
forming a memory cell in the first well;
forming a first transistor structure in the third well;
forming a second transistor structure in the fourth well; and
forming a third transistor structure in the fifth well.
2. A method according to claim 11, wherein the steps of forming the first
well, the second well, and the fourth well comprise the substeps of:
covering the semiconductor substrate with a first SiO.sub.2 film;
forming a first resist film to have first openings onto a first portion of
the semiconductor substrate;
ion-planting boron into the semiconductor substrate at the first openings
of the first resist film;
forming a second resist film to have second openings on a second portion of
the semiconductor substrate;
ion-implanting boron into the semiconductor substrate at the second
openings of the second resist film;
removing the second resist film from the semiconductor substrate; and
subjecting the semiconductor substrate to a first heat treatment for a
first time period.
3. A method according to claim 1, wherein the steps of forming the third
well and the fifth well comprise the substeps of:
forming a third resist film to have third openings on a third portion of
the semiconductor substrate;
ion-implanting phosphorus into the semiconductor substrate at the third
openings of the third resist film;
removing the third resist film from the semiconductor substrate; and
subjecting the semiconductor substrate to a second heat treatment for a
second time period.
4. A method according to claim 2, wherein the method of forming the memory
cell in the first well comprises the substeps of:
removing the first SiO.sub.2 film from the semiconductor substrate;
forming a first field oxide film on the semiconductor substrate;
forming a thick resist film on the semiconductor substrate;
forming a trench in a portion of the first well;
forming a second SiO.sub.2 film on the first well;
etching the trench and etching portions of the second SiO.sub.2 film
peripheral to the trench;
selectively removing a portion of the thick resist film located around the
trench;
forming a doped polysilicon layer of said first conductivity type impurity
on the semiconductor substrate;
subjecting the semiconductor substrate to a third heat treatment for a
third period of time;
removing the doped polysilicon layer from the semiconductor substrate;
forming a third SiO.sub.2 film on the semiconductor substrate; and
forming a polysilicon layer around and on the internal surface of the
trench.
5. A method according to claim 4, wherein the steps of forming the first
transistor structure, the second transistor structure, and the third
transistor structure comprise the substeps of:
forming a fourth SiO.sub.2 film of a predetermined thickness on the
semiconductor substrate;
forming a second polysilicon layer on the semiconductor substrate;
forming first conductivity type diffusion layers at first positions on the
semiconductor substrate;
forming second conductivity type diffusion layers at second positions on
the semiconductor substrate;
forming a fifth SiO.sub.2 film on the semiconductor substrate;
etching portions of the fifth SiO.sub.2 oxide film; and
forming aluminum wiring layers in said etched portions of the fifth
SiO.sub.2 film in respective first conductivity type diffusion layers and
second conductivity type diffusion layers.
6. A method of forming a semiconductor device with a second conductivity
type semiconductor substrate comprising the steps of:
forming a first well of a first conductivity type with a surface impurity
concentration in the semiconductor substrate;
forming, separately from the first well, a second well of the first
conductivity with a surface impurity concentration not higher than the
impurity concentration of the first well;
forming, separately from the semiconductor substrate, a third well of the
second conductivity type in the second well;
forming, separately from the first well and the second well, a fourth well
of the first conductivity type with an impurity concentration not higher
than the impurity concentration of the first well and an impurity
concentration not lower than the impurity concentration of the second
well;
forming, separately from the first well, the second well and the fourth
well, a fifth well of the second conductivity type in a semiconductor
substrate with a higher majority carrier concentration than the majority
concentration of the third well;
forming a memory cell in the first well;
forming a first transistor structure in the third well;
forming a second transistor structure in the fourth well; and
forming a third transistor structure in the fifth well.
7. A method of forming a semiconductor memory with a substrate of a P type,
comprising the steps of:
forming a first well of the P type in the substrate, said first well having
a predetermined impurity concentration;
forming, separately from the first well, a second well of an N type, said
second well having a predetermined impurity concentration;
forming, separately from the substrate, a third well of the P type in the
second well;
forming, separately from the first well and the second well, a fourth well
in the substrate;
forming, separately from the first well, the second well, and the fourth
well, a fifth well of the N type in the substrate;
forming a memory cell in the first well;
forming a first transistor structure in the third well;
forming a second transistor structure in the fourth well; and
forming a third transistor structure in the fifth well, wherein a potential
of the first well is set not higher than potentials of any of the second
to fifth wells.
8. A method of forming a semiconductor memory with a substrate of a P type,
comprising the steps of:
forming a first well of the N type in the substrate, said first well having
a predetermined impurity concentration;
forming a second well of the P type in the first well, said second well
having a predetermined impurity concentration;
forming, separately from the first well, the second well and the third
well, a fourth well in the substrate;
forming, separately from the first well, the second well, third well, and
the fourth well, a fifth well of the N type in the substrate;
forming a memory cell in the second well;
forming a first transistor structure in the third well;
forming a second transistor structure n the fourth well; and
forming a third transistor structure in the fifth well, wherein a potential
of the second well is set not higher than potentials of any of the second
to fifth well.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor memory with well structure, and
more particularly to a dynamic random access memory (DRAM) with CMOS
structure.
2. Description of the Related Art
In the conventional DRAM, CMOS structure has not been employed to
constitute the peripheral circuit of the memory cell array. Recently,
however, use of DRAMs with CMOS structure is growing.
FIG. 1 shows an example of the cross section of a
one-transistor/one-capacitor type DRAM with CMOS structure. In FIG. 1, 1
denotes a P-type Si body; 2 and 2*, P-wells formed in the same
manufacturing step; 3, an N-well; 4, an insulation film for a capacitor:
5, a capacitor electrode; 6, the gate insulation film of a transistor; 7,
the gate electrode of a transistor; 8 and 8*, N.sup.+ -type diffusion
layers (source, drain); 9, P.sup.+ -type diffusion layers (source, drain);
10, an insulation film; 11, an Al wiring layer; A, a memory cell section;
and B, the peripheral circuit thereof. P-well layer 2 is formed to have an
impurity concentration higher than that of P-type substrate 1.
Recently, it was found preferable to form a memory cell in a high impurity
concentration well in order to prevent a soft error.
In a conventional memory device, the impurity concentration of P-well 2*,
in which the memory cell is formed, is the same as that of P-well 2, in
which the peripheral circuit is formed. Therefore, it becomes necessary to
further increase the impurity concentration of the well for the memory
cell in order to suppress the soft error. From the circuit characteristic
point of view, it is not desirable to excessively increase the impurity
concentration of the well because it will increase the diffusion
capacitance between N.sup.+ -type layer 8* and P-well 2* and lower the
junction breakdown voltage. If the miniaturization technique of the IC
internal structure is further advanced, it becomes necessary to operate
the internal circuit on a voltage of 2 to 4 V, despite the fact that the
input/output circuit section of the IC is operated on a voltage of 5 V.
Thus, it becomes necessary to adequately change the impurity concentration
of the well, such as P-wells 2 and 2*, according to the difference in the
power source voltages. However, no practical technology has developed to
meet the requirement.
SUMMARY OF THE INVENTION
An object of this invention is to provide a semiconductor device which can
improve the trade-off between the soft error and the withstanding voltage
by utilizing different impurity concentration wells.
One of the features of this invention is that a plurality of wells of the
same conductivity type with different surface impurity concentrations are
formed in the semiconductor body and a memory cell or cells are formed in
that one of the wells which has the highest surface impurity
concentration.
Further, this invention has a second feature that first wells of a first
conductivity type (P) are formed in the semiconductor body (P or N), a
first well of a second conductivity type (N) is formed in one or more
wells selected from the first wells of the first conductivity type so as
to form a second well or wells of the first (P) or second (N) conductivity
type, and a memory cell or cells are formed in one of the wells having the
highest surface impurity concentration.
Thus, the basic feature of this invention is that the second conductivity
type well is formed in one or more selected wells of the first
conductivity type in order to partly neutralize the first conductivity
type impurity (P) with the second conductivity type impurity (N), thereby
providing the first conductivity type well (P.sup.+) having an impurity
concentration lower than the remaining first conductivity type well or
wells (P.sup.++) formed in the semiconductor body. Alternately, the basic
feature of this invention is that first conductivity type impurity is
further doped into one or more of the first conductivity type low impurity
concentration wells (P, P.sup.+) to form at least one first conductivity
type well of high impurity concentration (P.sup.++). In this way, there
can be obtained three different types of first conductivity type (P)
regions, that is, the semiconductor body (P) itself, the first
conductivity type well (P.sup.++) formed in the semiconductor body, and
the first conductivity type well (P.sup.+) having the first conductivity
type impurity concentration lowered by the doped second conductivity type
impurity. Further, in a case where two different types of second
conductivity type (N) wells are formed, or where two or more different
types of first conductivity type (P) wells are formed, first conductivity
type wells having more different degrees of impurity concentration can be
obtained. If the structure is applied to the dynamic random access memory,
it is preferable to form cells in one of the first conductivity type wells
having the highest impurity concentration. This is because the leak
current in the memory capacitor is required to be lowered and the soft
error is required to be minimum.
Further, in this invention, it is possible to divide an ion-implantation
step into a plurality of sub-steps and form a plurality of wells of
variously selected degrees of impurity concentration by controlling the
amount of impurity doped into each portion of the semiconductor substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of a semiconductor device with the
conventional well structure;
FIGS. 2A to 2C are cross sectional views showing the manufacturing process
of a semiconductor device with well structure according to one embodiment
of this invention;
FIGS. 3A to 3M are cross sectional views showing the manufacturing process
in a case where a trench type DRAM and the peripheral circuit thereof are
formed on an N-type substrate with well structure according to one
embodiment of this invention;
FIG. 4 is a cross sectional view showing the case in which the DRAM shown
in FIG. 3M is formed in a P-type substrate;
FIG. 5 is a cross sectional view of a modification of the DRAM shown in
FIG. 4;
FIG. 6 is a cross sectional view showing the case in which the DRAM shown
in FIG. 5 is formed in a P-type substrate;
FIG. 7 is a cross sectional view of another modification of the DRAM shown
in FIG. 4;
FIG. 8 is a cross sectional view of a modification of the DRAM which is
shown in FIG. 3M and in which various power source voltages are applied;
FIG. 9A shows a voltage generating circuit for supplying low voltage VCC to
the multi-power source DRAM shown in FIG. 8;
FIG. 9B shows a voltage generating circuit for supplying low voltage VBB to
the multi-power source DRAM shown in FIG. 8;
FIG. 10 is a cross sectional view showing the structure of a trench cell
type memory which can be applied to a DRAM utilizing this invention;
FIG. 11 is a cross sectional view showing the structure of a stacked cell
type memory which can be applied to a DRAM utilizing this invention;
FIG. 12 is a cross sectional view showing the structure of a stacked trench
(STT) cell type memory which can be applied to a DRAM utilizing this
invention;
FIG. 13 is a cross sectional view showing the structure of a planar cell
type memory which can be applied to a DRAM utilizing this invention;
FIG. 14 is a cross sectional view showing the structure of an SRAM which
can be applied to this invention; and
FIGS. 15A to 15E respectively show various CMOS inverters to which the
present invention can be applied.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
There will now be described an embodiment of this invention with reference
to the drawings. First, as shown in FIG. 2A, boron is ion-implanted using
the photolithographic method into that part of P-type semiconductor body
101 which is allocated for formation of first P-well region 102-1 of cell
area A in a dynamic RAM. In this case, the ion-implantation dosage amount
is 2.times.10.sup.14 cm.sup.-2 and the acceleration voltage is 100 kev. At
the same time, boron is also ion-implanted into that part which is
allocated for formation of second P-well region 102-2 of peripheral
circuit section B. After this, the semiconductor structure is subjected to
a first heat treatment at a temperature of 1190.degree. C. in an N.sub.2
gas atmosphere for about 6 hours so as to form P-well regions 102-1 and
102-2. FIG. 2A shows the semiconductor structure after the heat treatment.
In FIG. 2A, xxx indicates ion-implanted boron.
Next, phosphorus 104 is ion-implanted into a portion which is allocated for
formation of first N-well region 106 and internal portion 105 of second
P-type well 102-2 by using resist film 103 with the dose amount of
5.times.10.sup.13 cm.sup.-2 and at an acceleration voltage of 100 keV.
After this, the semiconductor structure is subjected to a second heat
treatment at a temperature of 1190.degree. C. in an N.sub.2 gas atmosphere
for about 4 hours. As a result, first P-well region (P.sup.++) 107 having
a surface impurity concentration of 5.times.10.sup.17 cm.sup.-3 is formed
in cell area A, and second P-well region (P.sup.+) 105 having a surface
impurity concentration of 2.times.10.sup.17 cm.sup.-3 is formed in the
N-channel region of peripheral circuit B.
Then, capacitor insulation film 108, storage electrode (polysilicon) 109,
N.sup.+ -type diffusion layer 110, gate electrode (polysilicon) 111 of a
write-in/readout transistor, and wiring layer (aluminum)112 for the bit
line are formed in cell area A. Further, gate electrode (polysilicon) 113
of an N-channel transistor. gate electrode (polysilicon) 114 of a
P-channel transistor, diffusion regions 115 and 116, and lead-out wiring
layer (aluminum) 117 for each electrode are formed in second P-well 105
and first N-well 106 of the peripheral circuit such as row/column decoders
and sense amplifiers, thus forming a CMOS DRAM as shown in FIG. 2C.
FIGS. 3A to 3M are cross sectional views showing the manufacturing process
used in a case where a trench type DRAM cell and the peripheral circuit
thereof are formed on an N-type substrate with a well structure according
to one embodiment of this invention.
As shown in FIG. 3A, resist film 303A is formed (patterning) on a
predetermined portion of N-type substrate 30I covered with SiO.sub.2 oxide
film 302A having a thickness of about 1000.ANG., and boron (P-type
impurity) is ion-implanted via the openings of resist film 303A with the
dose amount of 2.times.10.sup.13 cm.sup.-2 and at an acceleration voltage
of 100 kev. As the result of the ion-implantation process, P-type impurity
region 304A is formed in that portion of N-type substrate 30 which lies
under the openings of resist film 303A.
Then, as shown in FIG. 3B, new resist film 303B is formed to have openings,
and boron is ion-implanted via the openings of resist film 303B with the
dose amount of 3.times.10.sup.13 cm.sup.-2 and at an acceleration voltage
of 100 keV. As the result of the ion-implantation process, P-type impurity
region 304B and P-type impurity region (304A+304B) are formed in N-type
substrate 301 in addition to P-type impurity region 304A.
After this, resist film 303B is removed and the semiconductor structure is
subjected to a heat treatment in the N.sub.2 gas atmosphere at a
temperature of 1190.degree. C. for 6 hours. As a result, P-type impurity
regions 304A, 304B, and (304A+304B) are thermally diffused (drive-in
diffusion of P-well) to form P-well-2 having a low impurity concentration
(P), P-well-3 having a mid range impurity concentration (P.sup.+), and
P-well-1 having a high impurity concentration (P.sup.++), as shown in FIG.
3C.
Then, as shown in FIG. 3D, new resist film 303D is formed to have openings,
and phosphorus (N-type impurity) is ion-implanted via the openings of
resist film 303D with the dose amount of 3.times.10.sup.13 cm.sup.-2 and
at an acceleration voltage of 160 kev. As the result of the
ion-implantation process, N-type impurity regions 306D are formed in
N-type substrate 301 and P-well-2.
After this, resist film 303D is removed and the semiconductor structure is
subjected to a heat treatment in the N.sub.2 gas atmosphere at a
temperature of 1190.degree. C. for 4 hours. As a result, N-type impurity
regions 306D are thermally diffused (drive-in diffusion of N-well) to form
N-well-1 and N-well-2 in N-type substrate 301 and P-well-2, respectively,
as shown in FIG. 3E.
In the thermal diffusion process, N-type impurity of N-well-2 is partly
neutralized by P-type impurity of P-well-2, thus setting the impurity
concentration (N) of N-well-2 lower than that (N.sup.+) of N-well-1.
Then, oxide film 302A is removed and field oxide film 302F is formed as
shown in FIG. 3F. After this, thick resist film 302G is formed on the
semiconductor structure, and trench 310 for the memory cell capacitor is
formed in a predetermined portion of P-well-1 by means of a reactive ion
etching (RIE) method, or by use of a laser beam as shown in FIG. 3G.
Thereafter, a thermal oxide film (SiO.sub.2) having a thickness of about
200.ANG. is formed on the silicon substrate, where from the resist was
removed.
Next trench 310 and its peripheral thermal oxide film (SiO.sub.2) are
etched, a portion of field oxide film 302G around trench 310 is
selectively removed, and a doped polysilicon layer (not shown) having an
N-type impurity to be diffused into the surface area of trench 310 is
formed on the semiconductor structure. Then, the semiconductor structure
is subjected to a heat treatment to diffuse the N-type impurity from the
doped polysilicon layer formed thereon into the surface area of trench
310, thus forming diffusion layer 311 of the capacitor electrode, as shown
in FIG. 3H. The doped polysilicon layer is then removed after diffusion
layer 311 has been formed.
As shown in FIG. 3I, after heat oxide film 302H has been removed, extremely
thin SiO.sub.2 film 302I of 100 A is formed on surface substrate 301 which
has diffusion layer 311 formed therein. Oxide film 302I is used as
dielectric material of the memory capacitor. Polysilicon layer 312 of the
capacitor electrode is formed around and on the internal surface of trench
310, with part of dielectric oxide film 302I disposed between polysilicon
layer 312 and diffusion layer 311.
After this, the substrate surface is subjected to an oxidation process
whereby oxide film 302J having a predetermined thickness (for example,
200.ANG.) is formed on substrate 301z, and then polysilicon layer 313 to
be used as a gate electrode is formed on oxide film 302J as shown in FIG.
3J.
Next, N-type diffusion regions 308 which form source and drain regions are
formed on both sides of respective electrodes 313 of P-well-1 and P-well-3
by use of a resist film (not shown) as shown in FIG. 3K. Further, N-type
diffusion layers 308 are formed in the end portions of N-well-1 and
N-well-2.
Then, as shown in FIG. 3L, P-type diffusion layers 309 are formed in the
end portions of P-well-1 to P-well-3 by use of a resist film (not shown).
Further, P-type diffusion regions 309 which form source and drain regions
are formed on both sides of respective electrodes 313 of N-well-1 and
N-well-2.
Next, as shown in FIG. 3M, thick SiO.sub.2 film 302M is formed on the
entire surface of substrate 301, predetermined portions of second oxide
film 302M are etched out to form openings, and aluminum wiring layers 314
are selectively formed in connection with respective N-type diffusion
layers 308, P-type diffusion layers 309, and capacitor electrode 312, via
the openings.
In FIG. 3M, P-channel transistors, constituting a power source voltage
converting circuit, input/output circuit, input protection circuit, and
the like, are formed in N-well-1, and P-channel transistors, constituting
a sense amplifier and the like, are formed in N-well-2. Further, memory
cells, and N-channel transistors, constituting sense amplifier, word
driver, and the like, are formed in that one of P-well-1 to P-well-3 which
has the highest impurity concentration, that is, P-well-1. In addition,
the peripheral circuit and the like are formed in P-well-2.
Since N-well-2 is isolated from N-well-1 by means of P-well-2, voltages
applied to N-well-1 and N-well-2 can be set to have different voltage
levels. Further, it is possible that an N-well having an impurity
concentration higher than that of P-well-1 (P.sup.++) be provided in a
position which is not shown in the drawing.
FIG. 4 shows the case in which the DRAM shown in FIG. 3M is formed in a
P-type substrate. N-well-2 is formed in P-well-2 in FIG. 3M, but P-well-2
is formed in N-well-3 in FIG. 4.
FIG. 5 is a modification of the DRAM shown in FIG. 4. In FIG. 5, P-well-1
of the memory cell is formed in N-well-2, and N-well-1* in P-well-2 is
formed in the same step as that of N-well-1, or it is formed immediately
after the step of forming N-well-1.
FIG. 6 shows the case wherein the DRAM shown in FIG. 5 is formed in a
P-type substrate. The structure shown in FIG. 6 is similar to that of FIG.
5 except that the impurity conductivity type is inverted from either a
P-type impurity to N-type impurity or vice versa.
FIG. 7 is another modification of the DRAM shown in FIG. 4. In this
embodiment, N-well-1 has the highest impurity concentration (N.sup.++),
and a memory cell is formed therein. The peripheral circuit of the DRAM is
formed in N-well-2 which has a lower impurity concentration than N-well-1.
The term "highest impurity concentration" used in this embodiment refers
to N-type impurity, and therefore P-well-1 may be formed to have a P-type
impurity concentration which is higher than an N-type impurity
concentration of N-well-1, for example.
FIG. 8 is a modification of the DRAM shown in FIG. 3M to which various
power source voltages are applied. In this embodiment, external power
source voltage ExtvCC (+5 V) is applied to N-well-1 wherein P-channel
transistors constituting the input/output circuit and the like are formed.
Internal power source voltage IntvCC (+4 V) which is derived by means of
such a voltage generating circuit as shown in FIG. 9A is applied to
N-well-1 formed in P-well-2. With the structure shown in FIG. 8, the pn
junction between N-well-1 and P-well-2 is reversely biased because of the
potential difference between ExtVCC (+5 V) and IntVCC (+4 V) so that
N-well-1 is electrically isolated from P-well-2. Therefore, ExtVCC (+5 V)
can be applied to the input/output circuit which requires a relatively
high operation voltage, while at the same time IntVCC (+4 V or less) can
be applied to the internal circuit which is desirably operated at a low
voltage level.
Further, low negative voltage IntVBB (-2 V) which is applied to the memory
cell in P-well-1, for example, can be derived from such a voltage
generating circuit as shown in FIG. 9B.
Because the junction between P-well-1 and the N-type substrate is reversely
biased by the potential difference between ExtvCC (+5 V) or IntvCC (+4 V)
and IntvBB (-2 V), the low negative voltage IntvBB (-2 V) can thus be
used. In other words, since bias voltages of various levels can be applied
to the respective wells, the drain-source voltage of the transistor formed
in the well can be freely set.
Incidentally, CMOS inverters shown in FIGS. 15A to 15E can be applied to
the device of this invention.
The circuits shown in FIGS. 9A and 9B are disclosed in the following
documents:
1986 IEEE International Solid-State Circuits Conference
ISSCC 86/FRIDAY, FEBRUARY 21, 1986
SESSION XIX: DYNAMlC RAMs
pages 272-273
FAM 19.7: An Experimental 4Mb CMOS DRAM
Tohru Furuyama, Takashi Ohsawa, Yoji Watanabe, Hidemi Ishiuchi, Takeshi
Tanaka, Kazunori Ohuchi, Hiroyuki Tango, Kenji Natori, Osamu Ozawa
Toshiba Semiconductor Device Engineering Laboratory/VLSI Research Center
Kawasaki, Japan
FIG. 10 shows the structure of a trench cell type memory which can be
applied to a DRAM utilizing this invention. This structure is the same as
that of FIG. 3M (however, the conductivity type of substrate can be P or
N). Gate electrode 313 is used as a word line of the memory cell, and the
aluminum wiring layer connected to diffusion region 308 which lies on the
left side in FIG. 10 is used as a bit line of the memory cell.
Examples of trench type memories having structures other than those
mentioned above are disclosed in the following U.S. patent specifications:
______________________________________
U.S. Pat. No.: 4,672,410
Miura et al.
January 9, 1987
"SEMICONDUCTOR MEMORY
DEVICE WITH TRENCH
SURROUNDING EACH
MEMORY CELL"
U.S. Pat. No.: 4,673,962
Chatterjee et al.
January 16, 1987
"VERTICAL DRAM CELL AND
METHOD"
______________________________________
It should be understood that the contents disclosed in the above U.S.
patent specifications are incorporated in this application.
FIG. 11 shows the structure of a stacked cell type memory which can be
applied to a DRAM utilizing this invention. In the stacked cell type
memory, a memory capacitor is constituted by concave pot type polysilicon
storage node 312B and convex capacitor electrode 312A which is engaged
with a concave portion of storage node 312B, via an oxide film (dielectric
material), with a thickness of several tens .ANG..
FIG. 12 shows the structure of a stacked trench (STT) cell type memory
which can be applied to a DRAM utilizing this invention. The STT structure
can be attained by applying the stacked cell structure of FIG. 11 to the
trench structure of FIG. 10 The STT structure used in this invention is
suitable for DRAMs of a large memory capacity, for example, 16 M bits.
FIG. 13 shows the structure of a planar cell type memory which can be
applied to a DRAM utilizing this invention. This structure corresponds to
the cell structure in area A of FIG. 2C.
FIG. 14 shows the structure of an SRAM which can be applied to this
invention. In this embodiment, the drain wiring layer of the cell
transistor is formed in the form of two-layered structure having first
polysilicon layer (for internal wiring layer) 312F of low resistance and
second polysilicon layer (for load resistor) 312G of high resistance.
The Bipolar-CMOS technology disclosed in the following documents can be
applied as the conventional memory cell technology which can be used in
this invention:
______________________________________
IEEE JOURNAL OF SOLID-STATE CIRCUITS,
VOL. SC-22, No. 5, OCTOBER 1987
______________________________________
"An Experimental 1-Mbit BiCMOS DRAM"
pages 657-662
GORO KITSUKAWA, RYOICHI HORI, YOSHIKI
KAWAJIRI, TAKAO WATANABE, TAKAYUKI
KAWAHARA, KIYOO ITOH, YUTAKA KOBAYASHI,
MASAYUKI OOHAYASHI, KYOICHIRO ASAYAMA,
TAKAHIDE IKEDA, AND HIROSHI KAWAMOTO
408-IEDM86
"Advanced BiCMOS Technology for High Speed VLSI"
pages 408-411
T. Ikeda*, T. Nagano, N. Momma, K. Miyata,
H. Higuchi**, M. Odaka*, K. Ogiue*
Hitachi Research Laboratory, 4029, Kuji-cho,
Hitachi-shi, Ibaraki, 319-12, Japan
*Device Development Center, 2326, Imaicho, Oumeshi, Tokyo, 198, Japan
**Central Research Laboratory, Kokubunjishi, Tokyo, 185, Japan
Hitachi Ltd.
802-IEDM86
"Bipolar CMOS merged structure for
high speed M bit DRAM"
pages 802-804
Y. Kobayashi, M. Oohayashi, K. Asayama, T. Ikeda*
R. Hori** and K. Itoh**
Hitachi Research Laboratory, Hitachi, Ltd.,
Hitachi, Ibaraki, Japan, 319-12
______________________________________
*Device Development Center, Hitachi, Ltd., Oume, Tokyo, Japan 198
**Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo, Japan 185
Bipolar devices, which appear to be similar to but are substantially
different from the semiconductor device of this invention, are disclosed
in the following documents:
______________________________________
Physics and Technology of
Semiconductor Devices
page 209
A. S. GROVE
Fairchild Semiconductor, Palo Alto
University of California, Berkeley
John Wiley and Sons, Inc., New York .multidot. London .multidot. Sydney
Physics of Semiconductor Devices
SECOND EDITION
pages 192-197
S. M. Sze
Bell Laboratories, Incorporated
Murray Hill, New Jersey
A WILEY-INTERSCIENCE PUBLICATION
JOHN WILEY & SONS
New York # Chichester .multidot. Brisbane .multidot. Toronto .multidot.
Singapore
In general, the bipolar device includes high impurity concentration regions
and low impurity concentration regions, but they are substantially
different from the well of high impurity concentration (P.sup.++) and the
wells of lower impurity concentration (P.sup.+, P) of this invention. More
specifically, an active circuit element such as a memory cell is formed in
the high impurity concentration (P.sup.++) well in this invention, but in
the bipolar device, the high impurity concentration region (for example,
emitter) is used as part of the active circuit element. It should be
understood that the structure of this invention (combination of the high
and low impurity concentration wells) is essentially different from that
of the bipolar device (combination of low and high impurity concentration
regions such as the emitter and collector).
This invention is not limited to the embodiments described above, and can
be variously modified. For example, in the embodiment of FIG. 2C, a P-well
of low impurity concentration is formed in peripheral circuit section B by
using opposite conductivity type (P-type against N-type, for example)
impurity. However, it is possible to form a P-well of high impurity
concentration in cell section A by using the same conductivity type
(P-type for P-type, for example) impurity to enhance the impurity
concentration of the well. Further, in the embodiment, first N-well is
formed in the first P-well to form the second P-well with impurity
concentration lower than that of the first P-well. However, it is also
possible to form low and high impurity concentration N-wells by forming a
first N-well with impurity concentration higher than that of the first
P-well in the first P-well.
As described above, according to this invention, wells with various degrees
of impurity concentration can be easily formed, and therefore the wells
can be selectively used to attain desired elements. In the DRAM or the
like, a cell or cells can be formed in the well of the highest impurity
concentration and the peripheral circuit section can be formed in the well
with the impurity concentration next to the highest impurity
concentration, thus making it possible to significantly improve the
performance and characteristics of the semiconductor device.
Top